1. Field of the Invention
The invention is generally related to the use of pipe to transport hydrocarbons and more particularly to the pipe in pipe configurations used in the offshore environment.
2. General Background
The development and production of hydrocarbon reserves offshore may require certain levels of temperature to be maintained within the associated submarine pipeline and riser network to assure flowing of the transported fluids in the required state and operating pressure. To provide high levels of thermal insulation in submarine pipelines, typically at greater depths, a dual steel wall xe2x80x9cpipe-in-pipexe2x80x9d construction is used. The annular space between the outer pipe and inner pipe can be filled with an insulating layer of either a vacuum or a material of suitable insulating nature such as low-density polyurethane foam. In some designs bulkheads are installed at prescribed distances along the pipeline to compartmentalize flooding that may result due to a leak in the outer pipe. These bulkheads are typically made from a machined steel forging or plate ring weldment. The bulkheads may also be used to connect the inner pipe axially to the outer pipe for the purposes of transmitting axial loads between the inner and outer pipes associated with thermal expansion or other loading mechanisms.
Other means of insulating pipelines include the use of polymeric coatings to provide insulation. The insulating effectiveness of these materials is less than pipe in pipe. At greater depth, especially where high levels of insulation are required, the coatings are costly, if not impractical to install.
To date, the installation of pipe in pipe offshore entails the field jointing of sections of pre-fabricated pipe in pipe joints by welding. A pipe in pipe joint is fabricated from random length pipe of the required diameters and wall thickness issued from the mill. Multiple joint lengths of pipe may be assembled to create a longer pipe in pipe joint to minimize the number of pipe in pipe field joints needed to be field welded.
In all cases to date, the inner pipe is field jointed by butt-welding. The outer pipes are joined by one of two methods: the use of a sliding outer pipe or the use of sleeves/half-shells. A sliding outer pipe allows it to slip along the axis of the inner pipe after the inner pipe butt weld has been completed to permit the outer pipes to be aligned and welded as a butt weld. Where the slipping of the outer pipe is not possible due to bulkheads connecting the inner and outer pipe, the adjacent outer pipe joints are joined by using welded sleeves or half-shells to bridge the gap between the adjacent outer pipes that provide the required access to weld in the inner pipe.
For J-lay installation, the sliding outer pipe has evolved as the most cost-effective means of completing a pipe in pipe joint. Alternatives to the sliding outer pipe or sleeves are proprietary couplings (such as those by Snamprogetti) that merge the inner and outer pipe ends at both ends of a pipe in pipe joint into a single, thicker wall. This allows the joint between pipe in pipe joints to be only a single butt weld. The merged wall couplings must accommodate the stress changes that can occur in the pipeline as a result of the dramatic change in section properties that occur at the coupling. Use of the coupling also impacts the uniformity of insulating properties.
Jointing of single wall land and submarine pipelines using methods other than welding have been developed. These alternative methods include: threaded connections, cold forged joining and interference fit connections (such as the Jetair International Positive Seal Coupling System) that use a polymer to provide bonding and sealing properties for the coupling. Critical to the physical performance of this coupling is the dimensional tolerances and alignment of the pipes to permit the interference fit of the coupling.
In the case of submarine pipelines, their installation using either S-lay or J-lay installation techniques involves using spreads of specialist marine equipment and personnel. The high cost of operating these spreads requires the design of field joints of submarine pipelines to be practically completed in as little time as possible without compromising the integrity of the pipeline joint design. The current technology used to field joint pipe in pipe joints, while technically sound, is timely and costly.
The existing designs for field jointing of pipe in pipe pipelines result in the installed cost of submarine pipe in pipe pipelines to greatly exceed single wall pipelines. In the case of double walled field joints, significant cost arises during installation due to the need to complete two or more welds, including the required non-destructive testing and corrosion coating. In the case of merged wall couplings, higher cost results from the manufacturing and assembly of the coupling to the pipe in pipe joints and the greater wall thickness typically required to manage the stress levels in the coupling.
It is seen from the above that the current state of the art leaves a need for improvement in the design of pipe in pipe construction and the field jointing of pipe in pipe pipelines in order to provide certain technical and economic benefits.
The invention addresses the above need. What is provided is a pipe in pipe assembly where a coupling is used to join the ends of the inner pipe. The inner pipe is substantially centralized within the inner pipe in a manner that provides axial resistance but minimum restraint transverse to the axis of the assembly. This provides sufficient flexibility of the inner pipe to accommodate the necessary tolerances to permit the use of the coupling. The inner pipe may be centralized within the outer pipe by the use of foam injected between the inner and outer pipe and a corrugated web sleeve around the ends of the inner pipe. The inner pipe may also be centralized within the outer pipe by a corrugated web bulkhead that fits around the inner pipe and within the outer pipe.